High performance carbon fiber/epoxy composites are currently being considered for use in the aerospace industry for light weight structural parts to improve both payload capability and fuel efficiency. These epoxies have excellent fabricating characteristics and are the most commonly used matrix resins in applications not exceeding use temperatures of 280.degree.-350.degree. F. Such composites should preferably be capable of withstanding operating temperatures up to about 350.degree. F. for extended periods of time.
One disadvantage of many of the epoxy materials of the prior art employed to make these composites is their tendency to absorb moisture even at ambient conditions. Moisture pickup is accelerated in hot humid environments where composite part temperatures may approach such severe environmental conditions as 140.degree. F. and 75 to 100 percent relative humidity even when the composite part is not in actual use. The result is a significant lowering of the glass transition temperature of the composite part. Mechanical properties at the desired elevated use temperatures are severely reduced when this occurs since at temperatures increasingly above the Tg the thermoset composition becomes increasingly more plastic. Consequently, when such composites are exposed for brief periods to temperatures of about 300.degree. to about 350.degree. F. while being subjected to stress they often are adversely affected. Dimensional changes may also occur in parts which have absorbed water upon exposure to humid environments. These dimensional changes can cause severe difficulty in areas where close tolerances must be maintained.
An epoxy formulation currently used to prepare carbon fiber/epoxy prepregs as illustrated by U.S. Pat. No. 4,107,128 is 4,4'-tetraglycidylmethylenedianiline commonly known as MY-720 which has the structural formula: ##STR1##
This material is a high functionality, low viscosity, high reactivity epoxy with excellent wetting ability and provides a thermoset polymer exhibiting good initial high temperature (e.g., 350.degree. F.) stability but only moderate moisture resistance when cured with a commonly used curing agent such as 4,4'-diaminodiphenyl sulfone. Consequently, under environmental conditions of high humidity and high temperatures (e.g., 140.degree. F.) over extended periods of about 25 to 35 days the Tg of MY-720 cured with 4,4'-diaminodiphenylsulfone is reduced substantially. The ability of such thermoset polymers to withstand temperatures of from about 300.degree. to about 350.degree. F. is, therefore, substantially reduced.
Epoxies are believed to attract significant amounts of moisture because of the extensive presence of oxygen, and in some cases nitrogen in their structures. Hydrogen bonding in the matrix produces sites that bind water very tightly. In addition, the bound water is present in the matrix at points where "soft" easily plasticized elements such as the following are present: ##STR2##
In recent years, systems other than the epoxies have been examined and found to exhibit varying moisture resistance while maintaining reasonable property levels after exposure to hot-humid environments. Polysulfone thermoplastics pick up less water than the epoxies and their Tg is not significantly affected by this process. Polyimides pick up levels of water similar to epoxies but property levels are not significantly affected. Both materials have their drawbacks as prepregging resins. Polysulfone is a thermoplastic high viscosity material not readily handled by conventional prepregging equipment. Polyimides release volatiles during cure which can result in voids in the final part. Recently, addition polyimides have been made available (Keremid, Rhodia Inc., N.J.) which cure by an addition mechanism and not by condensation. These materials, however, are quite expensive and are known to contain microcracks when cured.
Modified epoxy systems, such as Hystl.TM. rubber modified epoxies (HME) have also known examined as a possibility for overcoming the moisture pick up problem. Hystl.TM. rubber typically comprises 18.8 percent, by weight, of carboxyl terminated 1,4-polybutadiene with the remainder being carboxyl terminated 1,2-polybutadiene. The higher the content of the rubber in the cured system, however, the greater the decrease in the elevated temperature properties thereof even in the absence of moisture. Hystl.TM. rubber when combined with various epoxies gives cured systems which show a significantly lower tendency to pick up water under hot-wet conditions than do epoxies alone.
Extensive work in this area was performed for the Air Force Materials Laboratory by the TRW Systems Group. The original HME resins were prepared by initially reacting Hystl.TM. C-1000 rubber with an epoxy novolak in methyl ethyl ketone to provide a block copolymer system.
Several phenolic novolak epoxies as well as cresol novolak epoxies were examined as possible epoxy candidates. The requirements which guided the selection of these epoxy resins included high epoxy functionality to yield an intense cross-linked network, solubility in a common solvent with the polybutadiene resin, and high temperature stability.
The choice of the 1,2-polybutadiene system instead of 1,4-polybutadiene was based on the desire to achieve a rigid network which would have greater strength, when subjected to temperatures of 300.degree.-350.degree. F., than the more flexible 1,4-system.
The presence of the carboxyl end groups on the rubber was thought to aid compatibility between the hydrophobic rubber and the hydrophilic epoxy. The Hystl.TM. rubber chosen (C-1000, M.W.=1,350) had a low molecular weight which allowed for minimal distance between epoxy groups so as to maintain adequate high temperature properties. The viscosity of the Hystl.TM. rubber at 45.degree. C. of 5000-20,000 cps was also in the useful range for handling with many epoxy systems. This material is commercially available from Dynachem Inc. (Irvine, Calif.).
No mention is made of the glycidylamine (MY-720) resin in their work.
The amine curing agent chosen was benzyldimethylamine (BDMA). Such tertiary amine cured systems, however, generally have limited temperature and chemical resistance. A peroxide was included for rubber cure. The epoxy finally chosen as the primary candidate was Ciba Geigy's ECN-1280 a multi functional epoxy cresol novolak which has a functionality of 5.1. This material is a solid with a melting point of 78.degree.-81.degree. C. and an epoxide equivalent weight of 230. This material is excessively brittle and consequently had to be modified by selecting a Hystl.TM. rubber having a high amount of 1,4-vinyl polybutadiene rubber and a low amount of the 1,2-isomer thereof. The final HME product resulting from this TRW effort had a maximum service temperature of about 275.degree. F. Even at 275.degree. F. the interlaminar shear strength was quite low.
Further work has been done in the area of HME systems since the initial TRW work. Hercules followed up this initial work and prepared materials capable of being hot-melt coated in a solventless system. Hercules, however, used a cycloaliphatic epoxy i.e., 2,3-epoxycyclohexylmethyl-2',4'-epoxycyclohexyl-carboxylate which they found to be compatible with Hystl.TM. rubber on mixing of the two materials. TRW had used a solvent system to combine the epoxy with the acid terminated rubber. Hercules also used more Hystl.TM. rubber in their formulation than did TRW (80 vs. 45%).
A solvent system presents an environmental pollution problem as well as a flammability problem. In addition the solvent must be removed during the curing procedure since it does not participate in the curing reaction. This is a further disadvantage since extensive vacuum is employed. Furthermore, any solvent not removed before final cure will result in gas pockets which create voids in the laminate. The presence of voids in the laminate contribute to reduced properties since they act as stress concentration points from which cracks in the laminate can form.
A more recent approach to making carbon fiber/epoxy composites conducted by TRW involved the use of a mixture of epoxies to achieve a compatible blend in combination with Hystyl.TM. rubber. The mixture of epoxies included DEN 438 epoxy novolak and ERE 1359 resorcinol epoxy resin and is cured with 4,4-diaminodiphenylsulfone. In addition a significant level of bis-(4-maleimidophenyl) methane was added to the mixture to obtain increased high temperature performance. The results of tests performed on the laminates prepared from the above composition at 350.degree. F. evidence a low interlaminar-shear strength even before exposure to moisture. It was concluded that the low shear strength at elevated temperatures was a result of the presence of the rubber. Attempts were made to improve the strength of the interface of the fiber to the rubber in the matrix to no avail.
A more detailed discussion of the work described above may be found in R. W. Vaughn and G. A. Zakrzewski, "Development of HME Laminating Resin," AFML-TR-75-194, October 1975; R. A. Johnson et al., "Low Flow, Low Pressure Prepregs," Air Force Materials Laboratory, interim reports for November 1976 and January 1977, IR-324-(1) and (2); C. E. Browning, "Selective Application of Materials for Products and Energy: HME Resin Matrix Systems" 23 Science of Advanced Materials and Process Engineering Series, pp. 541-51 (1978); and L. G. Adams and R. E. Hoffman, "Coupling Agents-HMS Resin System", AFML-TR-77-196 (1977).
Thus, although the HME systems described above reduced water pickup quite dramatically (50% reduction over typical epoxy systems), their shear strength at elevated temperatures is inferior to that achieved with currently used epoxy systems which are not modified with rubber in order to achieve moisture resistance such as the MY-720 epoxy resin cured with 4,4-diaminodiphenylsulfone. It is suggested that the poor performance with respect to shear strength at elevated temperatures of the HME Systems described above may be attributed to the use of epoxy compounds such as phenol or cresol epoxy novolaks which do not achieve a sufficiently high cross-linked density in the thermoset system.
In contrast, if one attempts to modify the MY-720 epoxy resin, which is capable of achieving a high cross-linked density with Hystl.TM. rubber and 4,4-diaminodiphenyl sulfone curing agent, a two phase system results due to the incompatibility of the components of the system.
Thus, the problems associated with providing a rubber modified epoxy system are two fold. If a solventless rubber modified epoxy system is desired for making prepregs, which is usually the case for reasons described herein, an epoxy system (epoxy resin and curing agent) must be used which is capable of forming a compatible mixture with the rubber. If a high level of elevated temperature properties is desired, the epoxy system must, in addition, be suitable for elevated temperature usage (e.g., possess a high initial dry T.sub.g which is not substantially reduced by either the presence of the rubber or the absorption of moisture). Epoxy systems which meet both of these requirements are quite limited.
Other examples of rubber and epoxy containing compositions may be found in U.S. Pat. Nos. 3,686,359; 3,947,522 and 4,020,030. None of these patents disclose the use of TGDDS in combination with a rubber.
Thus, there has been a continuing search for an epoxide compound and a thermosetting composition employing said epoxide which when cured can form castings or carbon fiber/epoxy composites meeting the above described two fold requirement.
The present invention is a result of this search.
It is therefore a general object of the present invention to alleviate the problems of the prior art.
It is a further object of the present invention to provide a novel epoxy compound and thermosetting composition prepared from the same.
It is another object of the present invention to provide a novel epoxy compound and curing agent which is compatible with a polybutadiene rubber.
It is a further object of the present invention to provide a polybutadiene rubber modified epoxy thermosetting composition which when cured exhibits enhanced moisture resistance and high elevated temperature properties as determined by an improved resistance against a reduction in the glass transition temperature of the cured composition when subjected to temperatures of about 140.degree. to about 180.degree. F. in a humid environment.
It is a still further object of the present invention to provide a polybutadiene rubber modified thermosetting epoxy composition which when cured exhibits enhanced moisture resistance and high elevated temperature properties as evidenced by an improved resistance against a reduction in the glass transition temperature of the cured composition when subjected to temperatures of about 140.degree. to about 180.degree. F. in a humid environment.
It is another object of the present invention to provide a carbon fiber/polybutadiene rubber modified epoxy composite having improved moisture resistance, and high elevated temperature properties as evidenced by an improved resistance against a reduction in the glass transition temperature of the composite when subjected to temperatures of about 140.degree. to about 180.degree. F. in a humid environment.
These and other objects and features of the invention will become apparent from the claims and following description.